Exhaust purification system for internal combustion engine

ABSTRACT

An exhaust purification system for an internal combustion engine capable of lean-burn driving comprises: an NSR catalyst disposed in an exhaust passageway; an SCR disposed downstream of the NSR catalyst; a NOx sensor, disposed downstream of the SCR, for producing output in response to NOx concentration; and rich-spike means for causing a rich-spike. When the NOx sensor has produced output indicative of a NOx concentration higher than a given NOx concentration, the system increases the amount of the NOx contained in the exhaust gas discharged during the rich-spike. Under a given high-load condition, the air-fuel ratio is made stoichiometric during the rich-spike caused at a particular timing. Under a given low-load condition, the amount of exhaust gas is increased during the rich-spike.

TECHNICAL FIELD

The present invention relates to exhaust purification systems forinternal combustion engines and particularly to an exhaust purificationsystem for an internal combustion engine having both of a NOx storagereduction catalyst and a NOx selective catalytic reduction.

BACKGROUND ART

A system is known in which the exhaust passageway of an internalcombustion engine is provided with a NOx storage-reduction catalyst(hereinafter referred to as an “NSR catalyst”), an example of which isdisclosed in JP-A-2001-271679. The NSR catalyst serves the function ofadsorbing the nitrogen oxides (NOx) contained in combustion gasesdischarged from the internal combustion engine, as well as serving thecatalytic function of purifying the NOx, hydrocarbons (HC), and thelike. When the internal combustion engine is being operated at a leanair-fuel ratio, a NOx-rich exhaust gas is discharged. Thus, the NSRcatalyst adsorbs this NOx, thereby preventing the NOx from flowing pastthe catalyst.

The NOx adsorbed by the NSR catalyst is purified at a particular timing.For instance, the above conventional system is designed to cause arich-spike by temporarily discharging unburnt gas components from theinternal combustion engine. This causes reactions within the catalystbetween the NOx stored by the catalyst and the discharged unburnt gascomponents.

When the rich-spike causes the internal combustion engine to discharge alarge amount of unburnt gas components, the exhaust gas flowing past theNSR catalyst becomes stoichiometric as long as there remains, in thecatalyst, NOx to be reduced by the unburnt gas components. After the NOxadsorbed by the catalyst has all been reduced, the exhaust gas becomesricher because some unburnt gas components start to flow past thecatalyst. Therefore, the above conventional system is designed to detectsuch an exhaust gas change into a rich one, which occurs downstream ofthe catalyst, by monitoring oxygen concentration or nitrogen oxideconcentration, and the timing of that detection is followed bytermination of the rich-spike. This prevents the rich-spike from beingcaused in an excessive manner, thereby also preventing deterioration offuel consumption.

-   Patent Document 1: JP-A-2001-271679-   Patent Document 2: JP-A-2009-114879

BRIEF SUMMARY OF THE INVENTION Problem to be Solved by the Invention

In the above conventional system, the NOx adsorbed by the NSR catalystis reduced by causing a rich-spike. However, during the rich-spike, partof the adsorbed NOx may occasionally flow through the catalyst. In otherwords, when the rich-spike causes unburnt gas components (i.e.,reductants) to be introduced into the NSR catalyst, the NOx adsorbed bythe catalyst is detached therefrom, causing reactions on the catalyst.At this time, however, part of the detached NOx will flow past thecatalyst without being purified by the catalyst.

To purify such escaping NOx, a NOx selective catalytic reduction(hereinafter referred to as an “SCR”) can be installed downstream of theNSR catalyst. The SCR can primarily adsorb the NH₃ generated within theNSR catalyst. Thus, the SCR is capable of selectively purifying the NOxthat has passed through the NSR catalyst using the adsorbed NH₃.

Note, however, that when the NH₃ stored by the SCR becomes scarce, theNOx purification performance of the SCR will decrease accordingly. Forthis reason, there has been a demand for clarifying the mechanism of NH₃generation and developing a system capable of generating large amountsof NH₃, so that the emission performance deterioration due to lack ofNH₃ can be prevented.

The present invention has been contrived to address the above issue, andan object of the invention is to provide an exhaust purification systemfor an internal combustion engine having both of an NSR catalyst and anSCR, the system having the capability of preventing the deterioration ofemission performance due to unadsorbed NOx.

Means for Solving the Problems

In accomplishing the above object, according to a first aspect of thepresent invention, there is provided an exhaust purification system foran internal combustion engine capable of lean-burn driving, the systemcomprising:

a NOx storage reduction catalyst (hereinafter referred to as an NSRcatalyst) disposed in an exhaust passageway of the internal combustionengine;

a NOx selective catalytic reduction (hereinafter referred to as an SCR)disposed downstream of the NSR catalyst;

an exhaust sensor, disposed downstream of the SCR, for producing outputin response to NOx concentration;

rich-spike means for causing a rich-spike at a particular timing duringthe lean-burn driving; and

NOx-quantity augmenting means for increasing the amount of NOx containedin exhaust gas discharged during the rich-spike, in the event that theexhaust sensor has produced output indicative of a NOx concentrationhigher than a given NOx concentration.

According to a second aspect of the present invention, there is providedthe system as described in the first aspect, wherein the NOx-quantityaugmenting means includes stoichiometric-spike means for achieving astoichiometric air-fuel ratio during the rich-spike caused at aparticular timing, in the event that the internal combustion engine isbeing operated under a given high-load condition.

According to a third aspect of the present invention, there is providedthe system as described in the first or second aspects, wherein theNOx-quantity augmenting means includes gas-quantity augmenting means forincreasing the amount of exhaust gas during the rich-spike, in the eventthat the internal combustion engine is being operated under a givenlow-load condition.

According to a fourth aspect of the present invention, there is providedthe system as described in any one of the first to third aspects,further comprising:

ignition-timing control means for controlling an ignition timing of theinternal combustion engine; and

floor-temperature acquisition means for acquiring a floor temperature ofthe NSR catalyst,

wherein the NOx-quantity augmenting means includes igniting-timingadvancing means for advancing the ignition timing before MBT timingduring the rich-spike, in the event that the internal combustion engineis being operated under a given high-load condition and also that thefloor temperature of the NSR catalyst is higher than a giventemperature.

According to a fifth aspect of the present invention, there is providedthe system as described in any one of the first to fourth aspects,wherein the NOx-quantity augmenting means includes slight-richnessachieving means for achieving a slightly rich air-fuel ratio during therich-spike, in the event that the internal combustion engine is beingoperated under a given low-load condition.

According to a sixth aspect of the present invention, there is providedthe system as described in any one of the first to fifth aspects,further comprising exhaust gas recirculation (EGR) means forrecirculating part of exhaust gas flowing inside the exhaust passagewayinto an intake passageway of the internal combustion engine,

wherein the NOx-quantity augmenting means includes means for prohibitingthe operation of the EGR means in the event that the internal combustionengine is being operated under a given low-load condition and also thatthe rich-spike is being caused.

According to a seventh aspect of the present invention, there isprovided the system as described in any one of the first to sixedaspects, further comprising multi-injection means for performingmultiple fuel injections during a single stroke,

wherein the NOx-quantity augmenting means includes means for operatingthe multi-injection means in the event that the internal combustionengine is being operated under a given low-load condition and also thatthe rich-spike is being caused.

According to a eighth aspect of the present invention, there is providedthe system as described in any one of the first to seventh aspects,further comprising prohibiting means for prohibiting the operation ofthe NOx-quantity augmenting means when the exhaust sensor has producedoutput indicative of a NOx concentration lower than a given NOxconcentration.

Effects of the Invention

According to the above first aspect of the invention, when the NOxconcentration on the downstream side of the SCR is higher than a givenNOx concentration, the NOx contained in the exhaust gas dischargedduring the rich-spike is increased in amount. The larger the amount ofNOx flowing into the NSR catalyst during the rich-spike, the more easilythe NSR catalyst produces a large amount of NH₃. Thus, in this aspect ofthe invention, a large amount of NH₃ can be supplied to the SCR when theNH₃ stored by the SCR is scarce. This effectively prevents thedeterioration of NOx emission performance due to lack of NH₃ in the SCR.

According to the above second aspect of the invention, when the internalcombustion engine is being operated under a given high-load condition,the air-fuel ratio is made stoichiometric during the rich-spike causedat a particular timing. Stoichiometric combustion leads to generation ofa large amount of NOx and also has less influence on the torque. Thus,this aspect of the invention allows a large amount of NOx to beintroduced into the NSR catalyst while at the same time satisfying theconditions required under a high-load condition.

According to the above third aspect of the invention, when the internalcombustion engine is being operated under a given low-load condition,the amount of exhaust gas is increased during the rich-spike. This gasamount increase during the rich-spike results in an increase in theamount of NOx flowing into the NSR catalyst. Also, when the internalcombustion engine is being operated under a low-load condition,increasing the gas amount during the rich-spike has less influence onfuel consumption. Thus, this aspect of the invention allows a largeamount of NOx to be introduced into the NSR catalyst while at the sametime preventing deterioration of fuel consumption.

According to the above fourth aspect of the invention, in the event thatthe internal combustion engine is being operated under a given high-loadcondition and also that the floor temperature of the NSR catalyst ishigher than a given temperature, the ignition timing is advanced duringthe rich-spike. Advancing the ignition timing during the rich-spike willresult in a decrease in exhaust gas temperature, but a stable torque canbe ensured with ease. Therefore, this aspect of the invention allows alarge amount of NOx to be introduced into the NSR catalyst while at thesame time satisfying the conditions required under a high-loadcondition.

According to the above fifth aspect of the invention, when the internalcombustion engine is being operated under a given low-load condition,the air-fuel ratio is made slightly rich during the rich-spike. Thus,this aspect of the invention allows an increase in the NOx concentrationof exhaust gas, thereby increasing the amount of NH₃ generated by theNSR catalyst.

According to the above sixth aspect of the invention, when the internalcombustion engine is being operated under a given low-load condition,exhaust gas recirculation is prohibited during the rich-spike. Thisaspect of the invention prevents the in-cylinder combustion temperaturefrom decreasing due to exhaust gas recirculation, thereby facilitatingNOx generation in an effective manner.

According to the above seventh aspect of the invention, when theinternal combustion engine is being operated under a given low-loadcondition, multiple fuel injections are performed during the rich-spike.Thus, this aspect of the invention facilitates formation of an air-fuelmixture and increases the in-cylinder combustion temperature, therebyfacilitating NOx generation in an effective manner.

According to the above eighth aspect of the invention, when the NOxconcentration on the downstream side of the SCR has become lower than agiven NOx concentration, the operation of the NOx-quantity augmentingmeans is prohibited. This aspect of the invention prevents execution ofunnecessary control operations when the SCR stores a sufficient amountof NH₃.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic to illustrate the system configuration ofEmbodiment 1 of the present invention.

FIG. 2 is a graph illustrating the relationship between the floortemperature of an SCR 18 and the NOx purification rate achieved by theSCR 1.

FIG. 3 is a graph illustrating the relationship between the exhaustair-fuel ratio and the concentrations of NH₃ generated by differentcatalysts.

FIG. 4 is a flowchart illustrating a routine executed in Embodiment 1 ofthe present invention.

FIG. 5 is a flowchart illustrating a routine executed in Embodiment 2 ofthe present invention.

MODE FOR CARRYING OUT THE INVENTION

Some embodiments of the present invention will now be described withreference to the accompanying drawings. Note that common elementsappearing in the drawings are given the same reference numerals and willnot be described in a repetitive manner. It should also be noted thatthe embodiments described below are not meant to limit the presentinvention.

Embodiment 1 [System Configuration of Embodiment 1]

FIG. 1 illustrates the system configuration of Embodiment 1 of thepresent invention. As illustrated in the figure, the system ofEmbodiment 1 includes an internal combustion engine (or simply anengine) 10. The exhaust side of the engine 10 is in communication withan exhaust passageway 12. Disposed within the exhaust passageway 12 is astarting catalyst 14 (hereinafter referred to as the SC 14), which is athree-way catalyst. Also, in the exhaust passageway 12, a NOx storagereduction (NSR) catalyst 16 is installed downstream of the SC 14. Inaddition, a NOx selective catalytic reduction 18 (hereinafter referredto as the SCR 18) is located downstream of the NSR catalyst 16 in theexhaust passageway 12.

The engine 10 tends to discharge HC and CO when the air-fuel ratio isrich and NOx when the ratio is lean. Under lean conditions, the SC 14reduces NOx (or purifies NOx into N₂) while adsorbing oxygen (O₂). Underrich conditions, in contrast, the SC 14 oxidizes HC and CO into H₂O andCO₂, respectively, while discharging oxygen. Also, under richconditions, the SC 14 causes the nitrogen contained in exhaust gas toreact with hydrogen, thereby generating ammonia (NH₃).

The NSR catalyst 16 adsorbs the NOx contained in exhaust gas under leanconditions and discharges the adsorbed NOx under rich conditions. TheNOx discharged under rich conditions is reduced by HC and CO. As withthe SC 14, the NOx reduction results in generation of NH₃.

The SCR 18 adsorbs the NH₃ generated by both of the SC 14 and the NSRcatalyst 16 under rich conditions. Under lean conditions, the SCR 18selectively reduces the NOx contained in exhaust gas, using the NH₃ as areductant. The use of the SCR 18 effectively prevents the atmosphericdischarge of the NH₃ and NOx that have passed through the NSR catalyst16.

The exhaust passageway 12 of the system of FIG. 1 has an air-fuel ratio(A/F) sensor 20 installed upstream of the SC 14. This A/F sensor 20 isused to detect the air-fuel ratio of exhaust gas discharged from theengine 10. The system also has oxygen (O₂) sensors 22 and 24 installedwithin the exhaust passageway 12, with the former sensor 22 beinglocated between the SC 14 and the NSR catalyst 16 and the latter sensor24 being located between the NSR catalyst 16 and the SCR 18. These O₂sensors 22 and 24 generate a signal in response to the oxygenconcentration of exhaust gas. An NOx sensor 26 is also installed in theexhaust passageway 12, positioned downstream of the SCR 18. The NOxsensor 26 generates signals in response to the NOx and NH₃concentrations of exhaust gas. Specifically, the NOx sensor 26 isdesigned to detect, under rich conditions, the NH₃ concentration ofexhaust gas passing through the SCR 18 and, under lean conditions, theNOx concentration of the exhaust gas.

As illustrated in FIG. 1, the system of Embodiment 1 also includes anelectronic control unit (ECU) 30. Various actuators are connected to theoutput of the ECU 30, such as a fuel injector and the like (notillustrated, though). Also, various sensors are connected to the inputof the ECU 30, examples of which include the above-mentioned A/F sensor20, O₂ sensors 22 and 24, NOx sensor 26, and other sensors for detectingthe operating conditions and state of the engine 10. Based on variousinput information, the ECU 30 controls the state of the system of FIG.1.

[System Operation of Embodiment 1] (Functionality and Operation of theNSR Catalyst 16)

First described are the functionality and operation of the NSR catalyst16. Under normal conditions, the ECU 30 runs the engine 10 at a leanair-fuel ratio (lean-burn driving). During lean-burn driving, largeramounts of oxidants such as NOx and the like are discharged thanreductants such as HC, CO, and so on. Thus, an attempt to purify thisexhaust gas using a three-way catalyst may not work because all the NOxcannot be purified for lack of reductants. Therefore, the system ofEmbodiment 1 has adopted the installation of the NSR catalyst 16 withinthe exhaust passageway 12. The NSR catalyst 16 has the function ofadsorbing NOx as nitrates such as Ba(No₃)₂ and the like. Accordingly,the system of Embodiment 1 effectively prevents the atmosphericdischarge of NOx even during lean-burn driving.

Note, however, that the NOx-adsorbing capability of the NSR catalyst 16may decrease with an increase in the amount of NOx adsorption. For thisreason, a long duration of lean-burn driving results in unadsorbed NOxflowing past the NSR catalyst 16. Thus, the system of Embodiment 1 isdesigned to perform rich-spike causing control by which the NOx adsorbedby the NSR catalyst 16 is detached therefrom on a regular basis. Morespecifically, at a particular timing when the adsorbing capability ofthe NSR catalyst 16 begins to decrease, the exhaust air-fuel ratio ofthe engine 10 is made temporarily rich (e.g., A/F=12). During thisrich-spike period, the exhaust gas contains large amounts of reductantssuch as HC, CO, H₂, and so on. When these reductants are introduced intothe NSR catalyst 16, the NOx adsorbed by the NSR catalyst 16 as nitratesis reduced to NO and detached from its bases. The detached NOx is thenpurified into N₂ and the like by the corresponding catalyst within theNSR catalyst 16. As above, by causing a rich-spike during lean-burndriving, the NOx adsorbed by the NSR catalyst 16 can be detachedtherefrom, thereby restoring the NOx-adsorbing capability In aneffective manner.

(NOx Purification by the SCR 18)

Next described is the functionality of the SCR 18. As stated above, theNOx-adsorbing capability of the NSR catalyst 16 can be restoredeffectively by causing a rich-spike. However, the rich-spike will causepart of the NOx detached from the NSR catalyst 16 to flow downstreamwithout being purified. Moreover, as also stated above, some NOx flowsdownstream without being adsorbed by the NSR catalyst 16 duringlean-burn driving. When such escaping NOx is discharged into theatmosphere, the emission performance may deteriorate.

For this reason, the system of Embodiment 1 includes the SCR 18 so as totreat the NOx that has passed through the NSR catalyst 16. As statedabove, the SCR 18 adsorbs the NH₃ generated by both of the SC 14 and theNSR catalyst 16 under rich conditions. The SCR 18 uses this adsorbed NH₃to selectively reduce or purify the NOx that has passed through the NSRcatalyst 16. This effectively prevents the atmospheric discharge of theNOx and deterioration of the emission performance.

It should be noted that, according to the present inventors, thereduction performed at the SCR 18 can be facilitated by making the floortemperature of the SCR 18 equal to or less than 500 degrees Celsius andpreferably equal to 300 degrees Celsius or thereabout. Accordingly, inthe system of Embodiment 1, the position of the SCR 18 is carefullychosen so as to make its floor temperature equal to about 300 degreesCelsius. This effectively prevents NOx from flowing past the SCR 18.

[Distinctive Operations Performed in Embodiment 1]

With reference now to FIGS. 2 and 3, distinctive operations ofEmbodiment 1 will be described. As stated above, when a rich-spike iscaused during lean-burn driving of the engine 10, the NOx adsorbed bythe NSR catalyst 16 is purified into N₂ and the like, and NH₃ is alsogenerated as an intermediate byproduct of the purification process. Thegenerated NH₃ is adsorbed by the SCR 18 located downstream of the NSRcatalyst 16 and used for the purification of NOx.

Note here that when the NH₃ adsorbed by the SCR 18 becomes scarce, theNOx flowing into the SCR 18 may not be purified effectively. To addressthis issue, the present inventors have conducted a study on themechanism of NH₃ generation. FIG. 2 is a graph illustrating therelationship between the floor temperature of the SCR 18 and the NOxpurification rate achieved by the SCR 18. In the graph, Curve (a)represents the NOx purification rate achieved when the NSR catalyst 16was installed upstream of the SCR 18, while Curve (b) represents the NOxpurification rate achieved when a three-way was installed upstream ofthe SCR 18 in place of the NSR catalyst 16. As can be seen, theinstallation of the three-way catalyst resulted in a drastic decrease inthe NOx purification rate of the SCR 18. The result implies that theeffective use of a catalyst having a larger number of bases like the NSRcatalyst 16 is suited for the purpose of increasing the amount of NH₃generation.

The present inventors have also conducted studies on how the exhaustair-fuel ratio affects the amount of NH₃ generation. FIG. 3 is a bargraph illustrating the relationship between the exhaust air-fuel ratioand the concentrations of NH₃ generated by different catalysts. In thegraph, Bars (a) represent the concentrations of NH₃ generated by the NSRcatalyst 16, while Bars (b) represent the concentrations of NH₃generated by a three-way catalyst.

As the graph reveals, when the air-fuel ratio is somewhere betweenstoichiometric and slightly rich, NH₃ is generated not only by the NSRcatalyst 16 but also by the three-way catalyst having a smaller numberof bases. This would be a result of the large amount of NOx inherentlycontained in a stoichiometric gas. Thus, the result of FIG. 3 impliesthat the effective use of a stoichiometric atmosphere is suited for thepurpose of increasing the amount of NH₃ generation.

Therefore, to generate NH₃ in an effective manner, it is important toeffectively utilize a stoichiometric atmosphere and introduce a largeamount of NOx into the NSR catalyst 16. Accordingly, when the NH₃adsorbed by the SCR 18 has been judged scarce, the present embodimentwill perform the following control operations, so that the NOx containedin the exhaust gas discharged during a rich-spike can be increased inamount.

(Gas-Quantity Augmenting Control During a Rich-Spike)

An increase in exhaust gas quantity will lead to an increase in NOxamount. Thus, in the present embodiment, the gas quantity is increasedduring a rich-spike. This allows a large amount of NOx to be introducedinto the SC 14 and the NSR catalyst 16, thereby increasing the amount ofNH₃ generation in an effective manner. This control operation, however,will also result in an increase in the amount of fuel injection,compromising fuel consumption. For this reason, Embodiment 1 is designedto perform this control operation only when the engine 10 is beingoperated under a low-load condition. This allows the amount of NH₃generation to be increased effectively while at the same time preventingdeterioration of fuel consumption.

(Stoichiometric-Spike Control)

As stated above, a large amount of NOx is contained in a stoichiometricgas. Thus, in Embodiment 1, the air-fuel ratio is made stoichiometricduring a rich-spike caused at a particular timing. This means thatstoichiometric-spike is exercised during the time interval of causing arich-spike; consequently, a large amount of NOx can be introduced intothe SC 14 and the NSR catalyst 16. Note, however, that in terms of theamount of NH₃ generation, the above-mentioned gas-quantity augmentingcontrol is more desirable than this stoichiometric-spike control, butstill, the latter control is also advantageous in that it has lessinfluence on the torque. Therefore, Embodiment 1 is designed to exercisethe stoichiometric-spike control only when the engine 10 is beingoperated under a high-load condition. This allows the amount of NH₃generation to be increased effectively while at the same time satisfyingthe torque required under a high-load condition.

[Specific Operations Performed in Embodiment 1]

Next, specific operations performed in Embodiment 1 will be describedwith reference to FIG. 4. FIG. 4 is a flowchart illustrating a routineexecuted by the ECU 30. Note that the routine of FIG. 4 is repeatedduring lean-burn driving of the engine 10.

The routine of FIG. 4 starts with Step 100 in which a judgment is madeas to whether the NH₃ stored by the SCR 18 is scarce or not. Morespecifically, the ECU30 judges whether a value detected by the NOxsensor 26 is larger than a given concentration (e.g., 2 ppm) or not.When the condition that the detected value >2 ppm is not met, the NH₃stored by the SCR 18 is judged not scarce, resulting in the repetitionof this Step 100.

When, on the other hand, the above condition that the detected value >2ppm is met in Step 100, the purification performance of the SCR 18 isjudged to have decreased, and the routine proceeds to Step 102 in whicha judgment is made as to whether the engine 10 is being operated under agiven high-load condition. More specifically, the ECU 30 judges whetheror not the conditions that engine load KL>60 and engine revolution speedNE>2,800 rpm are met. When those conditions are met (KL>60 and NE>2,800rpm), it is determined that the engine 10 is being operated under ahigh-load condition, and the routine proceeds to Step 104 in which thestoichiometric-spike control is exercised. In this step, the air-fuelratio is made stoichiometric during a rich-spike caused at a particulartiming.

When, on the other hand, the above conditions that KL>60 and NE>2,800rpm are not met in Step 102, the routine proceeds to Step 106 in which ajudgment is made as to whether the engine 10 is being operated under agiven low-load condition. Specifically, the ECU 30 judges whether or notthe conditions that engine load XL<60 and engine revolution speedNE<2,800 rpm are met. When those conditions are not met (XL<60 andNE<2,800 rpm), the routine returns to Step 102 to perform it again.When, on the other hand, the conditions that XL<60 and NE<2,800 rpm aremet in Step 106, it is determined that the engine 10 is being operatedunder a low-load condition, and the routine proceeds to Step 108 inwhich the gas-quantity augmenting control is performed during arich-spike.

After the execution of Step 104 or 108, a judgment is made in Step 110as to whether the emission level of NOx has decreased or not.Specifically, the ECU 30 judges whether or not a value detected by theNOx sensor 26 is lower than a given concentration (e.g., 2 ppm). Whenthis condition that the detected value <2 ppm is not met, the NH₃ storedby the SCR 18 is judged still scarce, and the routine returns to Step102 to perform it again. When, on the other hand, the condition that thedetected value <2 ppm is satisfied in Step 110, it is determined thatthe SCR 18 has adsorbed a sufficient amount of NH₃, resulting intermination of this routine.

As described above, in the event that the NH₃ stored by the SCR 18 isscarce and also that the engine 10 is being operated under a givenhigh-load condition, the system of Embodiment 1 exercises thestoichiometric-spike control. This increases the amount of NOx inexhaust gas while at the same time satisfying the torque required undera high-load condition.

Further, in the event that the NH₃ stored by the SCR 18 is scarce andalso that the engine 10 is being operated under a given low-loadcondition, the system of Embodiment 1 increases the amount of exhaustgas during a rich-spike. This increases the amount of NOx in the exhaustgas while at the same time preventing deterioration of fuel consumption.

Furthermore, when the NOx concentration on the downstream side of theSCR 18 has decreased to a given value, the system of Embodiment 1prohibits the stoichiometric-spike control as well as the gas-quantityaugmenting control during a rich-spike. This effectively prevents thedeterioration of drivability and fuel consumption due to unnecessarycontrol operations.

As stated above, in Embodiment 1, the amount of exhaust gas is increasedduring a rich-spike in the event that the NH₃ stored by the SCR 18 isscarce and also that the engine 10 is being operated under a givenlow-load condition. It should be noted however that there are othermethods as well for increasing the amount of NOx in exhaust gas. Forinstance, if the engine 10 includes an exhaust gas recirculation (EGR)system, it is possible to prohibit an EGR operation during a rich-spikewhile at the same time exercising (or without exercising) thegas-quantity augmenting control. This prevents the in-cylindercombustion temperature from decreasing, thereby further facilitating thegeneration of NOx. Also, if the engine 10 includes a multi-injectionfuel injector capable of performing several injections during asingle-stroke, it is possible to perform multiple injections during arich-spike while at the same time exercising (or without exercising) thegas-quantity augmenting control. This facilitates formation of anair-fuel mixture and effectively increases the in-cylinder combustiontemperature, thereby further facilitating the generation of NOx.

Moreover, while Embodiment 1 is designed to increase the amount ofexhaust gas during a rich-spike in the event that the NH₃ stored by theSCR 18 is scarce and also that the engine 10 is being operated under agiven low-load condition, it is instead possible to exercise theslight-richness achieving control of Embodiment 2 described later, inplace of the gas-quantity augmenting control.

The following should be noted. In the above-described embodiment 1, TheNSR catalyst 16, the SCR 18, and the NOx sensor 26 correspondrespectively to the “NSR catalyst,” “SCR,” and “exhaust sensor” of thefirst aspect of the present invention described earlier. Also, in theabove-described embodiment 1, the “NOx-quantity augmenting means” of thefirst aspect of the invention is implemented by the ECU 30 executingStep 104 or 108 described above.

Further, in the above-described embodiment 1, the “stoichiometric-spikemeans” of the second aspect of the invention is implemented by the ECU30 executing Step 104.

Furthermore, in the above-described embodiment 1, the “gas-quantityaugmenting means” of the third aspect of the invention is implemented bythe ECU 30 executing Step 108.

Moreover, in the above-described embodiment 1, the “prohibiting means”of the eighth aspect of the invention is implemented by the ECU 30executing Step 110 described above.

Embodiment 2 [Distinctive Features of Embodiment 2]

With reference to FIG. 5, Embodiment 2 of the present invention willnext be described. The system of Embodiment 2 can be implemented byusing the hardware configuration of FIG. 1 and by having the ECU 30execute the routine of FIG. 5 described below.

When the NH₃ stored by the SCR 18 is scarce, Embodiment 2 is designed toperform control operations different from those performed in Embodiment1, so that the NOx contained in the exhaust gas discharged during arich-spike can be increased in amount. The following is a detaileddescription of those control operations of Embodiment 2.

(Ignition-Timing Advancing Control During a Rich-Spike)

Advancing the ignition timing stabilizes combustion. Thus, doing soensures a relatively stable torque with ease, even when the engine 10 isbeing operated under high-load conditions. In Embodiment 2, therefore,when the engine 10 is being operated under high-load conditions, theignition timing is advanced before the MBT timing during a rich-spike.This ensures a stable torque and effectively increases the amount of NOxin exhaust gas without compromising fuel consumption. Note, however,that advancing the ignition timing will result in a decrease in exhaustgas temperature. Accordingly, Embodiment 2 is designed to perform thisignition-timing advancing control only when the NSR catalyst 16 has beenwarmed up enough. This effectively prevents the NSR catalyst 16 frombecoming less active.

(Slight-Richness Achieving Control During a Rich-Spike)

As stated above, a larger amount of NOx is contained in a stoichiometricgas than in a rich gas. Thus, in Embodiment 2, the air-fuel ratio ismade slightly rich (e.g., A/F=13.5) during a rich-spike. Consequently, alarge amount of NOx can be introduced into the SC 14 and the NSRcatalyst 16. However, if this control is performed under high-loadconditions, a stable torque may not be obtained. Accordingly, Embodiment2 is designed to perform the slight-richness achieving control only whenthe engine 10 is being operated under low-load conditions. Thiseffectively increases the amount of NH₃ generation while preventingdeterioration of drivability.

[Specific Operations Performed in Embodiment 2]

Next, specific operations performed in Embodiment 2 will be describedwith reference to FIG. 5. FIG. 5 is a flowchart illustrating a routineexecuted by the ECU 30. Note that the routine of FIG. 5 is repeatedduring lean-burn driving of the engine 10.

The routine of FIG. 5 starts with Step 200 in which a judgment is madeas to whether the NH₃ stored by the SCR 18 is scarce or not. Morespecifically, the ECU30 performs the same operation as in Step 100 ofFIG. 4. When the condition that a detected value >2 ppm is not met, theNH₃ stored by the SCR 18 is judged not scarce, resulting in therepetition of this Step 200.

When, on the other hand, the above condition that the detected value >2ppm is met in Step 200, the purification performance of the SCR 18 isjudged to have decreased, and the routine proceeds to Step 202 in whicha judgment is made as to whether the engine 10 is being operated under agiven high-load condition. More specifically, in Step 202, the ECU 30performs the same operation as in Step 102 of FIG. 4. When theconditions that KL>60 and NE>2,800 rpm are met, it is determined thatthe engine 10 is being operated under a high-load condition, and theroutine proceeds to Step 204 in which a judgment is made as to whetherthe NSR catalyst 16 has been warmed up enough or not. Specifically, inStep 204, the ECU 30 judges whether or not the floor temperature T_(NSR)of the NSR catalyst 16 has reached a given temperature indicative of thecompletion of the warm-up (e.g., 350 degrees Celsius). When thecondition that the floor temperature T_(NSR)>350 degrees Celsius is notmet, it is determined that exercising the ignition-timing advancingcontrol described below will result in a decrease in the catalyticactivity of the NSR catalyst 16, and the routine then returns to Step202 to perform it again.

When, on the other hand, the condition that the floor temperatureT_(NSR)>350 degrees Celsius is met in Step 204, the routine proceeds toStep 206 in which the ECU 30 performs the ignition-timing advancingcontrol. More specifically, the ignition timing is advanced before theMBT timing during a rich-spike.

When, in Step 202, the conditions that KL>60 and NE>2,800 rpm are notmet, the routine proceeds to Step 208 in which a judgment is made as towhether the engine 10 is being operated under a low-load condition ornot. Specifically, in Step 208, the ECU 30 performs the same operationas in Step 106 of FIG. 4. When the conditions that KL<60 and NE<2,800rpm are not met, the routine returns to Step 202 to perform it again.When, on the other hand, the conditions that KL<60 and NE<2,800 rpm aremet in Step 208, it is determined that the engine 10 is being operatedunder a low-load condition, and the routine proceeds to Step 210 inwhich the slight-richness achieving control is performed during arich-spike. Specifically, the air-fuel ratio is made slightly rich(e.g., A/F=13.5) during a rich-spike.

After the execution of Step 206 or 210, a judgment is made in Step 212as to whether the emission level of NOx has decreased or not.Specifically, in Step 212, the ECU 30 performs the same operation as inStep 110 of FIG. 4. When the condition that a detected NOx value <2 ppmis not met, the NH₃ stored by the SCR 18 is judged still scarce, and theroutine returns to Step 202 to perform it again. When, on the otherhand, the condition that the detected value <2 ppm is satisfied in Step212, it is determined that the SCR 18 has adsorbed a sufficient amountof NH₃, resulting in termination of this routine.

As described above, in the event that the NH₃ stored by the SCR 18 isscarce, that the engine 10 is being operated under a given high-loadcondition, and also that the NSR catalyst 16 has been warmed upcompletely, the system of Embodiment 2 exercises the ignition-timingadvancing control. This increases the amount of NOx in exhaust gas whileat the same time satisfying the torque required under a high-loadcondition.

Further, in the event that the NH₃ stored by the SCR 18 is scarce andalso that the engine 10 is being operated under a given low-loadcondition, the system of Embodiment 2 makes the air-fuel ratio slightlyrich during a rich-spike. This increases the amount of NOx in theexhaust gas while at the same time preventing deterioration ofdrivability.

Furthermore, when the NOx concentration on the downstream side of theSCR 18 has decreased to a given value, the system of Embodiment 2prohibits the ignition-timing advancing control as well as theslight-richness achieving control. This effectively prevents thedeterioration of drivability and fuel consumption due to unnecessarycontrol operations.

As stated above, in Embodiment 2, the air-fuel ratio is made slightlyrich during a rich-spike in the event that the NH₃ stored by the SCR 18is scarce and also that the engine 10 is being operated under a givenlow-load condition. It should be noted however that there are othermethods as well for increasing the amount of NOx in exhaust gas. Forinstance, if the engine 10 includes an exhaust gas recirculation (EGR)system, it is possible to prohibit an EGR operation during a rich-spikewhile at the same time exercising (or without exercising) theslight-richness achieving control. This prevents the in-cylindercombustion temperature from decreasing, thereby further facilitating thegeneration of NOx. Also, if the engine 10 includes a multi-injectionfuel injector capable of performing several injections during afour-stroke cycle, it is possible to perform multiple injections duringa rich-spike while at the same time exercising (or without exercising)the slight-richness achieving control. This facilitates formation of anair-fuel mixture and effectively increases the in-cylinder combustiontemperature, thereby further facilitating the generation of NOx.

Moreover, while Embodiment 2 is designed to make the air-fuel ratioslightly rich during a rich-spike in the event that the NH₃ stored bythe SCR 18 is scarce and also that the engine 10 is being operated undera given low-load condition, it is instead possible to exercise thegas-quantity augmenting control of Embodiment 1 during the rich-spike,in place of this slight-richness achieving control.

The following should be noted. In the above-described embodiment, theNSR catalyst 16, the SCR 18, and the NOx sensor 26 correspondrespectively to the “NSR catalyst,” “SCR,” and “exhaust sensor” of thefirst aspect of the present invention described earlier. Also, in theabove-described third embodiment, the “NOx-quantity augmenting means” ofthe first aspect of the invention is implemented by the ECU 30 executingStep 206 or 210 described above.

Further, in the above-described embodiment 2, the “ignition-timingadvancing means” of the fourth aspect of the invention is implemented bythe ECU 30 executing Step 206.

Furthermore, in the above-described embodiment, the “slight-richnessachieving means” of the fifth aspect of the invention is implemented bythe ECU 30 executing Step 210.

Moreover, in the above-described embodiment, the “prohibiting means” ofthe eighth aspect of the invention is implemented by the ECU 30executing Step 212 described above.

DESCRIPTION OF REFERENCE CHARACTERS

-   10: internal combustion engine (engine)-   12: exhaust passageway-   14: start catalyst (SC)-   16: NOx storage-reduction catalyst (NSR catalyst)-   18: NOx selective catalytic reduction (SCR)-   20: A/F sensor-   22: O₂ sensor-   24: O₂ sensor-   26: NOx sensor-   30: ECU (electronic control unit)

1. An exhaust purification system for an internal combustion enginecapable of lean-burn driving, the system comprising: a NOx storagereduction (hereinafter referred to as an NSR catalyst) catalyst disposedin an exhaust passageway of the internal combustion engine; a NOxselective catalytic reduction (hereinafter referred to as an SCR)disposed downstream of the NSR catalyst; an exhaust sensor, disposeddownstream of the SCR, for producing output in response to NOxconcentration; rich-spike means for causing a rich-spike at a particulartiming during the lean-burn driving; and NOx-quantity augmenting meansfor increasing the amount of NOx contained in exhaust gas dischargedduring the rich-spike, in the event that the exhaust sensor has producedoutput indicative of a NOx concentration higher than a given NOxconcentration, wherein the NOx-quantity augmenting means includesgas-quantity augmenting means for increasing the amount of exhaust gasduring the rich-spike, in the event that the internal combustion engineis being operated under a given low-load condition.
 2. The systemaccording to claim 1, wherein the NOx-quantity augmenting means includesstoichiometric-spike means for achieving a stoichiometric air-fuel ratioduring the rich-spike caused at a particular timing, in the event thatthe internal combustion engine is being operated under a given high-loadcondition.
 3. (canceled)
 4. The system according to claim 1, furthercomprising: ignition-timing control means for controlling an ignitiontiming of the internal combustion engine; and floor-temperatureacquisition means for acquiring a floor temperature of the NSR catalyst,wherein the NOx-quantity augmenting means includes igniting-timingadvancing means for advancing the ignition timing before MBT timingduring the rich-spike, in the event that the internal combustion engineis being operated under a given high-load condition and also that thefloor temperature of the NSR catalyst is higher than a giventemperature.
 5. The system according to claim 1, wherein theNOx-quantity augmenting means includes slight-richness achieving meansfor achieving a slightly rich air-fuel ratio during the rich-spike, inthe event that the internal combustion engine is being operated under agiven low-load condition.
 6. The system according to claim 1, furthercomprising EGR means for recirculating part of exhaust gas flowinginside the exhaust passageway into an intake passageway of the internalcombustion engine, wherein the NOx-quantity augmenting means includesmeans for prohibiting the operation of the EGR means in the event thatthe internal combustion engine is being operated under a given low-loadcondition and also that the rich-spike is being caused.
 7. The systemaccording to claim 1, further comprising multi-injection means forperforming multiple fuel injections during a single stroke, wherein theNOx-quantity augmenting means includes means for operating themulti-injection means in the event that the internal combustion engineis being operated under a given low-load condition and also that therich-spike is being caused.
 8. The system according to claim 1, furthercomprising prohibiting means for prohibiting the operation of theNOx-quantity augmenting means when the exhaust sensor has producedoutput indicative of a NOx concentration lower than a given NOxconcentration.
 9. An exhaust purification system for an internalcombustion engine capable of lean-burn driving, the system comprising: aNOx storage reduction (hereinafter referred to as an NSR catalyst)catalyst disposed in an exhaust passageway of the internal combustionengine; a NOx selective catalytic reduction (hereinafter referred to asan SCR) disposed downstream of the NSR catalyst; an exhaust sensor,disposed downstream of the SCR, for producing output in response to NOxconcentration; rich-spike device for causing a rich-spike at aparticular timing during the lean-burn driving; and NOx-quantityaugmenting device for increasing the amount of NOx contained in exhaustgas discharged during the rich-spike, in the event that the exhaustsensor has produced output indicative of a NOx concentration higher thana given NOx concentration, wherein the NOx-quantity augmenting deviceincludes gas-quantity augmenting device for increasing the amount ofexhaust gas during the rich-spike, in the event that the internalcombustion engine is being operated under a given low-load condition.